1. Field of the Invention
The invention relates to a receiver having an RF input for applying an RF input carrier thereto, which is coupled to a phase-locked loop (PLL) having a signal path incorporating a phase detector, a loop filter, a controlled amplifier and a controlled oscillator, and a signal generator for generating a local auxiliary pilot and a pilot detector for detecting the local auxiliary pilot, the pilot detector having a first input coupled to the signal path of the phase-locked loop between an output of the phase detector and a control input of the controlled oscillator, a second input coupled to an output of the signal generator, and an output coupled to a control input of the controlled amplifier via a selection device.
2. Description of the Related Art
A receiver of this type is known per se, for example, from European Patent Application no. 89 20 29 62, (corresponding to U.S. Pat. No. 5,093,930).
The known receiver is a directly mixing AM PLL synchronous receiver in which a synchronous detector is used for a synchronous amplitude detection of a modulation signal which is amplitude-modulated on the RF input carrier. The local mixing carrier required for this synchronous detection is obtained by means of the controlled oscillator incorporated in the phase-locked loop and should be phase-synchronous with the RF input carrier for a correct synchronous detection. The local mixing career is therefore also referred to as the local in-phase carrier. An accurate phase equality between the RF input carder and the local in-phase carrier is obtained if the local quadrature carrier supplied by the controlled oscillator to the phase detector differs accurately 90.degree. in phase from the RF input carrier.
As is known, the phase detector supplies a phase difference signal which in practice does not only depend on the phase difference deviating from 90.degree. between the RF input carrier and the local quadrature carrier, but also on the amplitude or field strength of the RF input carrier. To prevent the loop transfer properties, such as the gain, bandwidth, phase shift and stability of the loop, from varying with the amplitude of the RF input carrier, an automatic control of the loop gain is realized in the known AM PLL synchronous receiver. The control signal required for this automatic control is obtained by means of a mixer stage in which the RF input carrier is mixed with a local in-phase carrier which is amplitude-modulated with the auxiliary pilot. A desired mixing product is then obtained whose frequency corresponds to that of the auxiliary pilot and whose amplitude varies with that of the RF input carrier. After a low-pass selection of this desired mixing product, a conversion or demodulation of this mixing product with the original auxiliary pilot is effected in the pilot detector, which results in a dc signal varying with the field strength variations of the RF input carrier, this de signal being subsequently applied as a gain control signal to the control input of the controlled amplifier.
However, due to oscillator radiation and other effects of parasitic crosstalk, the unmodulated local carriers supplied by the controlled oscillator occur with a crosstalk-dependent amplitude at the RF input of the receiver. These parasitic oscillator signals are multiplied in the last-mentioned mixer stage by the local in-phase carrier of the controlled oscillator, amplitude-modulated with the auxiliary pilot, resulting in a parasitic mixing product having a frequency which is equal to that of the auxiliary pilot. This parasitic mixing product is superimposed on and passes, together with the desired mixing product, the low-pass selection and gives rise to a parasitic dc offset of the gain control signal at the output of the pilot detector. This parasitic dc offset of the gain control signal increases with the frequency of the oscillator signal. Consequently, the automatic gain control is disturbed to an extent increasing with the frequency, which results in a reduction, increasing with the frequency, of the dynamic range of the receiver.
Moreover, phase errors between the local careers regenerated with the controlled oscillator and the RF input carrier, due to, for example, spreads in the free running frequency of the controlled oscillator, varying ambient factors such as ageing and temperature variations, do not affect the gain control signal of said known receiver. Therefore, the variations in the loop transfer due to these phase errors, hereinafter also referred to as slowly varying or static phase errors, are not compensated for by the known gain control circuit.